1. This schematic helps significantly: http://www.dfrobot.com/wiki/images/a/a0/RoMeo_Schematic.pngThe manual for this thing is horrible. I think it is cute how they mention that S1-S5 are on an analog input, but don't bother to show how to use it or mention which Pin.

2. This Arduino varient takes advantage of the 2 (little know) additional Analog inputs provided by the ATmega, which is why A7 looks odd in the example code.

3. As noted, you can't PWM Pin 13. However a little known bug (feature?) of AnalogWrite is that if you AnalogWrite values greater than 127 to a non-PWM pin, it will write a HIGH. Regardless, you should be using digitalWrite(13, HIGH) for Pin 13.

4. Based on the schematic, S1 will return a value of 0 while S2 will return a value of 700mV. This means analog values between 0 and ~140 are probably S1. So you might change your "if-statement" to be:

As mike suggests, the easiest thing to do is write a sketch that prints out of the value of the analogRead. Then go through and press each button. Just remember that you are reading an analog input. So if one of the buttons returns 143 today it might return 141 or 144 tomorrow.